1. Field of the invention
The invention is based on an actuator for a control valve having a control circuit which sets the actuator in accordance with the reference value predetermined by a master system control, having a main piston which slides in a main cylinder, having a drive volume, which can be pressurized with oil in a controlled manner, on one side of the main piston and having an oil-filled buffer volume on the other side of the main piston.
2. Discussion of background
An actuator for the activation of a control valve with which, for example, the steam supply to a turbine of a power station system can be controlled, has a main piston which on the one hand is impinged on by spring force, and on the other hand pressurized with oil. Given falling pressure of the oil, the spring force reliably closes the control valve, as a result of which the steam supply is disconnected. By this means, it is ensured that the turbine does not get out of control if the pressure of the oil should happen to drop. The oil pressure in a drive volume which acts on the main piston and activates the control valve via the piston is generated by an electro-hydraulic transducer. Given a movement of the control valve in the direction of opening, oil under pressure is fed into the drive volume, however since this movement occurs relatively slowly, comparatively small cross-sections are sufficient for the feeding of the oil. However, a closing movement of the control valve has to occur at a speed which is approximately ten times higher. This requires a comparatively rapid emptying of the drive volume which, however, cannot be achieved through the small cross-sections of the oil feed.
In addition, it is clear that, due to the increase in turbine power, the control valves and thus also the actuators which activate them have to be of larger and stronger design. A corresponding proportional enlargement of actuators leads to arrangements with comparatively large amounts of oil under pressure for their activation. With commercially available valves, quantities of oil of this kind can only be controlled with difficulty, in addition the dynamic performance of the actuator also suffers with increasing size.